1. Field of the Invention
The present invention relates to an antenna apparatus and a portable communication apparatus, and more particularly is suitably applied to a so-called multi-band portable communication apparatus made correspondent to at least two or more types of wireless communication systems, for example, different in used wireless communication frequency.
2. Description of the Related Art
In recent years, portable communication apparatuses have a tendency to become insufficient in the number of lines with a rapid spread if using lines for a single wireless communication system. Accordingly, it is considered that two types of wireless communication systems using different frequency bands are jointly used to ensure the required number of lines and there has been developed a terminal capable of using two types of wireless communication systems by means of a single portable communication apparatus with a significant progress in downsizing and weight-lightening technique.
On the other hand, there has been a problem of the effect of the electromagnetic waves irradiated from the portable communication apparatus on the human body during the telephone call. The absorption factor of electromagnetic waves absorbed by a specific region (chiefly head) of a human body per time and per mass during the telephone call has been defined as a local average Specific Absorption Rate (SAR) and it has been required to suppress a maximum of local average SARs below a prescribed value.
As shown in FIG. 1, reference numeral 1 denotes a portable communication apparatus developed for the purpose of suppressing a maximum of local average SARs below a prescribed value as a whole. A circuit substrate which is not illustrated necessary for wireless communication is housed inside a casing which is not illustrated. The circuit substrate is covered with a shield case 2 as a ground member.
With respect to this portable communication apparatus 1, covering the circuit substrate housed inside with a shield case 2 prevents a transmitter/receiver circuit or various other circuits packaged on the circuit substrate from being badly affected by each other and from badly affecting an antenna 4 or other equipment.
Besides, the inside circuit substrate is so arranged as to generate a transmission signal of a given signal type by a transmitter/receiver circuit for communications with a base station, transmit it from the antenna 4 via an antenna feeder part 3 to a base station and demodulate the reception signal received by the antenna 4 after taking it in via the antenna feeder part 3.
Here, the antenna 4 comprises a rod-shaped rod antenna, for example, made of a conductive wire rod. Other than this, it is possible to utilize a helical antenna formed by winding a conductive wire rod in a helical or various types of antennas including a complex of the above antennas in an expansion and contraction type.
With this antenna 4, the relevant antenna 4 alone does not operate as an antenna, and a high-frequency current flows from the antenna feeder part 3 also into a ground conductor of the circuit substrate or into a shield case 2 with the result that the portable communication apparatus 1 operates as an antenna as a whole.
The portable communication apparatus 1 is so arranged as to measure a local average SAR during the telephone call as shown in FIG. 2. And at this time it has been confirmed that a spot at which the local average SAR indicates a maximum (hereinafter, referred to as hot spot) lies near an ear coming into contact with a speaker 7.
This is considered to be because the speaker 7 of a portable communication apparatus 1 is used in contact with the ear of a human body or because the ground conductor of a circuit substrate present behind the speaker 7 or the shield case 2 operates as part of the antenna to irradiate electromagnetic waves.
Such being the case, with the portable communication apparatus 1 shown in FIG. 1, a conductive flat plate 5 is located at a position slightly floating from the top surface 2A of the shield case 2 opposite the speaker 7 which is not illustrated so as to be parallel with the top surface 2A.
At this time, in the conductive flat plate 5, one end is short-circuited to the shield case 2 by a short-circuiting conductor 6 while the other end is made to be electrically opened from the shield case 2 toward above as represented by the arrowhead a. The length L1 from the short-circuit end to the open end is chosen equal to a quarter of the wavelength xcex/4 of a wireless communication frequency.
Thereby, in the portable communication apparatus 1, the impedance between the conductive flat plate 5 and the shield case 2 becomes almost xe2x80x9c0xe2x80x9d at the short-circuit end, but the impedance at the open end approaches to an infinity and as a result, a high-frequency current becomes difficult in flowing from the vicinity of the antenna feeder part 3 to the conductive flat plate 5 or the shield case 2.
Incidentally, it has experimentally proven in the conductive flat plate 5 that the impedance at the open end reaches a maximum when the length L1 from the short-circuit end to the open end is chosen equal to a quarter of the wavelength xcex/4 of a wireless communication frequency.
Accordingly, in the portable communication apparatus 1, a high-frequency current becomes difficult in flowing from the vicinity of the antenna feeder part 3 to the conductive flat plate 5 or the shield case 2, so that the radiative quantity of electromagnetic waves irradiated from the conductive flat plate 5 and the shield case 2 is reduced, thereby enabling the local average SAR near an ear to be reduced.
However, in a portable communication apparatus 1, since the length L1 from the short-circuit end to the open end in the conductive flat plate 5 is determined by a wireless communication frequency, it was difficult to respectively reduce the local average SAR corresponding to two types of wireless communication systems different in wireless communication frequency unless two types of conductive flat plates respectively different in length L1 from the short-circuit end to the open end are provided.
In view of the foregoing, an object of this invention is to provide an antenna apparatus and a portable communication apparatus capable of respectively reducing the quantity of electromagnetic waves absorbed by a human body corresponding to at least two or more wireless communication systems different in wireless communication frequency, even in case of any wireless communication frequency.
The foregoing object and other objects of the invention have been achieved by the provision of an antenna apparatus and a portable communication apparatus which comprise a grounded conductor; a conductive flat plate with one end electrically short-circuited to the grounded conductor and the other end electrically opened to the grounded conductor; and a dielectric, inserted in between the conductive flat plate and the grounded conductor, with the electrical length from the one end to the other end of the conductive flat plate made identical for at least two or more types of wireless communication frequencies on the basis of the frequency dispersibility.
Thereby, the electrical length from the one end to the other end of the conductive flat plate becomes identical for two or more types of wireless communication frequencies depending on a dielectric, so that in case of performing communication via an antenna element for two or more types of wireless communication frequencies, the impedance at one end of a single conductive flat plate can be made almost equivalent for any wireless communication frequency to suppress the surface current, thus enabling the quantity of electromagnetic waves absorbed by a human body.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.